Various methods for producing polystyrene are known, generally using emulsion polymerization, suspension polymerization, dispersion polymerization, and so on. For example, Japanese Patent Unexamined Publication No. Hei 2-14222 teaches a polystyrene production method using emulsion polymerization. Methods for producing expandable polystyrene using suspension polymerization are disclosed in various patented literatures, including Japanese Patent Unexamined Publication No. Showa 46-15112, Japanese Patent Unexamined Publication No. Hei 5-317688, U.S. Pat. Nos. 5,559,202, 2,652,392, U.K. Patent No. 1188252, and Korean Patent Publication No. 10-1999-0024927.
However, the known methods for producing expandable polystyrene beads using suspension polymerization present several disadvantages, such as a large particle size distribution, an unwanted grade, creation of sludge or wastes, requiring a number of classification steps using a screening apparatus in order to yield pure products.
Another method for producing extruded polystyrene particles and expandable polystyrene beads is to use extrusion by adding palletized polystyrene particles with a blowing agent. However, in the course of adding the blowing agent to polystyrene, it is necessary to control dispersion of melted products and heat generated. In addition, during extrusion, the molecular weight of polystyrene beads may be reduced and additives used may be decomposed. In particular, the polystyrene beads produced by extrusion and expansion cannot fabricate molded articles having low density of 20 kg/m3 or less.
Korean Patent Publication No. 10-2005-0111820 describes a method of preparing expandable polystyrene beads containing graphite, comprising suspending pellets produced by re-extruding general polystyrene pellets together with graphite in a reactor and impregnating by adding a blowing agent. According to this method, since extruded pellets are further extruded and the resultant re-extruded pellets are subjected to suspension and impregnation, extrusion processing costs of micropellets considerably increase. In addition, it is quite difficult to take additional treatment measures for achieving high functionality, such as high expandability, strength, or thermal stability.
In the expanding of the obtained expandable polystyrene beads containing graphite particles, conventional expanding conditions can be employed without any particular limitations. The expandable polystyrene beads containing graphite particles can be expanded to have a cell size of about 70-300 microns, and have various merits and advantages, including high expandability, excellent heat insulation properties, strength and absorptiveness.